Search

We investigate the kinetics of competitive adsorption of binary gas mixtures in restrictive nano-porous materials by simulating the uptake dynamics of the mixture inside a longitudinal pore via a Kinetic Monte Carlo scheme. Adsorption/desorption events of the gas occur only through the ends of the pore, and the gas transport inside the pore is limited to single-file diffusion. Under these conditions, we show how the difference between the adsorption/desorption rates of species, together with the transport restrictions imposed by the single-file diffusion, leads to eventual trapping of the weaker species at equilibrium. In this situation, while the total distribution of the mixture along the pore is uniform at equilibrium, the distributions of the individual components are not. Remarkably, this kinetic trapping causes a deviation from the composition of the adsorbed mixture expected at equilibrium.

We present the results of a combined experimental and computational study of CO2adsorption on purified HiPco nanotubes. Isotherms were measured at six temperatures between 147 and 207 K, below the bulk triple point for CO2. Unlike the case of other adsorbates on HiPco nanotube bundles, adsorption isotherms at corresponding temperatures do not reveal the presence of any resolvable substeps. The isosteric heat values derived from the measured isotherms are lower than the latent heat of sublimation for most of the loadings, with the exception of a narrow range at very low coverage. Results from grand canonical Monte Carlo simulations show that this is due to the much larger contribution of the CO2–CO2interactions (owing mostly to the presence of the electrostatic component) that greatly exceeds the size of the gas–surface interaction as the coverage increases beyond the monolayer. Measurements of the kinetics of adsorption show that the equilibration time increases with sorbent loading, which is typical of systems with relatively larger adsorbate–adsorbate interactions.

When a given gas can adsorb onto a surface with multiple binding energies, temperature-programmed desorption (TPD) spectra may become relatively more complicated to analyze, making more difficult the extraction of energy and kinetic parameters from the experimental results. Here we present results from kinetic Monte Carlo simulations used to investigate gas desorption from external surfaces of carbon nanotube bundles. By varying the initially adsorbed coverage and keeping track of the particles as they desorb and diffuse across the surface, we identify specific features on the TPD spectra originated by the effects of the surface heterogeneity on the desorption kinetics.

We investigate the kinetics of adsorption of gases on the external surfaces of a carbon nanotube bundle by simulating the elemental processes that take place during equilibration in a kinetic Monte Carlo scheme. By modeling the adsorbing surface as a lattice composed by three or more linear chains of sites characterized by different binding energies, we are able to describe the overall kinetic behavior of the system as well as to identify the processes responsible for the observed behavior. The presence of sites with different binding energies introduces specific paths of adsorption, as particles use the weaker binding sites as temporary intermediate states before being adsorbed on stronger binding sites. This mechanism greatly increases the adsorption rate of the stronger sites with two main consequences as compared with our previous results for homogeneous surfaces: (1) the time evolution of the coverage is nonexponential, and (2) the overall equilibration time as a function of coverage deviates from the linear dependence.

We explore the adsorption behavior of Ne and CF4in as-produced carbon nanohorn aggregates using adsorption measurements and computer simulations. Despite the difference in specific surface area observed for these gases, the isotherms for both gases show the presence of two substeps before monolayer completion. With the aid of computer simulations, we conclude that this universal behavior is the result of adsorption in interstitial spaces of varying dimension in the aggregates.

We have explored the adsorption kinetics of argon and methane on chemically opened and on as-produced carbon nanotubes. We monitored the evolution of the adsorbate pressure after each gas dose was added to the cell during the performance of adsorption isotherms. We found significant differences between the equilibration times measured on these two groups of nanotubes. We propose an explanation for our experimental observations that makes use of recent computer simulations conducted for similar adsorbed systems.

The kinetics of gas uptake on different regions of carbon nanotube bundles is investigated by means of a kinetic Monte Carlo scheme. A lattice-gas description is used to model the adsorption of particles on a one-dimensional chain of sites under two types of dynamics:  (a) external kinetics, in which the chain is on the bundle's external surface directly exposed to the gas, and (b) pore-like kinetics, expected to occur inside the tubes and interstitial channels, where adsorption occurs via gas diffusion from the ends. From the time evolution of the coverage at a fixed temperature, equilibration times are obtained as a function of chemical potential (or amount adsorbed). The equilibration time of the external phase decreases linearly as the coverage increases toward monolayer completion; the rate at which this occurs strongly depends on the ratio between the binding energy and the temperature. Because of this dependence, unexpectedly long waiting times can be observed for very low coverages in systems with relatively high binding energies. The adsorption rate in pore-like phases is typically 2 orders of magnitude slower than that of external phases. We show how this large disparity between adsorption rates can hinder the observation of adsorption inside the tubes and in the interstitial channels during measurements of adsorption isotherms.